Photon correlation spectroscopy (or dynamic light scattering, DLS) measures the time resolved signal scattered from particle suspensions. The relaxation time of the sample is determined from the correlation function of the scattered signal from which the particle size distribution can be estimated. The technique works best when each particle in suspension scatters light from the illuminating light beam (e.g. laser) only and not light that has already been scattered from other particles. At high concentrations multiple scattering tends to degrade the technique.
Within a small range of backscattered angles, multiply scattered signals may have an almost identical relaxation time (from which the particle size is calculated) to the singly scattered signal.
An existing technique (which may be termed non-invasive back scatter, or NIBS) uses a moving lens to place an illuminating laser optical path and a backscatter detection optical path into a variable position within a sample cuvette, as shown in FIGS. 1 and 2. The intersection of the illuminating optical path and the detection optical path may be termed the detection region.
When the sample is turbid (i.e. has a high concentration of particles), the detection region can be placed near to the cell wall, which significantly reduces multiple scattering due to the foreshortened illumination path length within the sample. In addition, a backscatter angle may be selected at which multiply scattered signals have a similar relaxation time to singly scattered signals, as already described.
Moving the detection region within the cell is advantageous, and it is also advantageous to maintain a selected angle of detection throughout the range of movement, so as to combine both benefits mentioned above.
At low particle concentration, the detection region may be moved toward the cell centre, or at least away from the static scattering contribution from the wall. Whilst the static scattering contribution from the wall may be negligible compared with the scattering contribution from particles in a high concentration sample, such static scattering from the wall may be a source of uncorrelated noise (or even static reference signal), for low concentration samples. The static scattering contribution from the wall may therefore decrease signal to noise ratio. The static scattering increases the correlogram baseline and thence reduces its intercept, which is a measure of the signal-to-noise of the measurement. Moving the detection region away from the cell wall may therefore improve the signal to noise ratio.
In the low sample concentration limit, DLS suffers from number fluctuations, whereby the scattered signal varies because of the fluctuation in the number of particles within the detection region, in addition to the contribution to the scattering from the Brownian motion of the particles. However, it may not be practical to simply expand the size of the detection beam to accommodate more particles, because this may increase the size of the beam out of a single coherence area. The highest signal-to-noise measurements using DLS may rely on measurement from within a single coherence area.
The signal to noise ratio of the correlogram is generally interpreted from the intercept of the correlogram and the y-axis. In order to maximize this value a single mode fibre may be used in the detection optical path, to select out a single spatial frequency from the ‘image’ of the speckle field. Simply increasing the size of the detection optical path may result in non-optimal coupling into such a fibre or may collect light from more than one coherence area, which may reduce the signal to noise ratio.
A method and apparatus for solving or ameliorating at least some of the above mentioned problems is desirable.